The project examines the effect of the administration to hospitalized patients of different doses of different antibacterial agents upon the prevalence of different bacterial species and antibiotypes among isolates obtained from them. It exploits an unprecedented resource fortuitously created by concurrent comprehensive surveys of drug usage and of isolate antibiotype which include in common 4,400 patient admissions on two wards over a 7 year period. All data from both surveys has already been collected and computer filed and some of the programs for its analysis have been developed. The magnitude and detail of the data thus available should provide a more discriminating analysis than hitherto available of the processes governing emergence of resistance to antibacterial agents, and hence contribute to a rational basis for antibiotic usage policy.